Triac Control (Simple 1 Ph. AC Motor)

Do you have a circuit diagram you could post or could describe what the 100V capacitor is actually doing? If it is just a filter capacitor then a 250V replacement should be OK (assuming it fits), but there are other considerations that may need to be addressed depending on the circuit and how the capacitor is actually being used.

can anyone give me an output waveform/voltage at motor terminals for this circuit.

If you make sure to not change the capacitance value of the capacitor when you replace it with a higher voltage rating, then you will have no effect on how the circuit functions.

What you have is a simple speed control circuit for an AC motor. Now the voltages you will find at the common node of the motor and the triac will be very difficult to read because the back EMF of the motor will be adding voltages in ways that will depend on the mechanical load on the motor.

It might be easier if you tell us what you know about how a triac functions. (I don't mean the sub-atomic Physics but if you can...) A key thing that you should know is that a diac is literally a pair of diodes in parallel and reverse phased. They guarantee that there must be at least +/- 0.6V voltage across them before current can conduct.

So i think my doubt is cleared.That in this case the capacitor voltage rating has no effect on the functioning of circuit. I have studied about Triac.Its having similar characteristics of 2 SCR connected in antiparallel way(bidirectional).I want an output waveform for(230VAC) this circuit without motor connected so that i can understand how the Gate triggering is done to achieve required voltage. I guess the capacitor is charged and when i adjust the preset... please explain this part. What happens when preset resistance is maximum w.r.t phase & diac and when its minimum.

Ok good, you understand how an SCR functions. Now remember that a capacitor has a fixed impedance (X_c) for a fixed frequency. Prior to the triac being triggered on each cycle of power, the gate input impedance will be fairly high. So the variable resistor combination and capacitor make a RC attenuator that has two distinct characteristics, attenuation of the AC signal (the capacitor does not remain charged) and a phase shift lag of when the peak voltage occurs compared to the source voltage. When this attenuated voltage briefly exceeds the diac voltage drop and the triac gate threshold voltage the triac turns ON and the gate becomes a low input impedance. Remember that just like an SCR, a triac turns OFF only once current stops flowing though the rectifier. So by changing the value of R with the potentiometer the phase delay will significantly change. Thus you can control on what part of the AC waveform the triac starts to conduct the waveform. Now you might worry that you cannot get all of the available power to the motor because there will always be some part of the leading edge of each half cycle that cannot be applied to the motor. This is technically true but is a much less significant component than anticipated because the motor is an inductive load. The current through an inductor lags behind the applied voltage anyway.

The AC attenuation circuit.Some pure electronic stuff i think.I have to study on this topic. In my circuit i am getting an output range of 40V to 75V for 175V input.Which doesnt have slight impact on the motor.Its all because i didnt get specific components for the circuit. There are many commutation processes can you tell what rectifier does here?

Let me look in to my power electronics basics.Yesterday i wondered of seeing RC triggering in my graduation lab syllabus.